1. Field of the Invention
The present invention relates generally to fuel rods for a nuclear reactor and, more particularly, is concerned with an apparatus and method for producing multi-level heat input for nuclear fuel rod weld formation using a single current level power supply.
2. Description of the Prior Art
A nuclear reactor core is composed of a multiplicity of fuel assemblies with each fuel assembly comprised of a plurality of nuclear fuel rods. A typical nuclear fuel rod is manufactured by loading a plurality of nuclear fuel pellets and a plenum spring into a cladding tube and applying bottom and top end plugs to opposite ends of the cladding tube. Typically, the end plugs are welded to the ends of the tube in an atmosphere of inert gas which provides a suitable shield or cover gas for performance of arc welding. Argon, helium, and a helium-argon mixture are examples of suitable cover gases, with helium ordinarily being the preferred cover gas. Conventional welding operations performed in the manufacture of nuclear fuel rods are disclosed in U.S. Patents to Boyko et al (3,683,148), Fink et al (3,725,635), Boyko et al (3,842,238 and 4,003,788) and Duncan et al (4,075,454).
Arc welding of end plugs to the tube ends must be carried out with care and precision to avoid various types of weld defects, such as cracking, porosity, distortion of the tube adjacent to the inner extension of the end plug, and a reduction in the thickness of the tube adjacent to the weld, commonly referred to as I.D. undercutting. For reducing the frequency of the I.D. undercutting weld defect, a low heat first pass and high heat second pass technique has been used. This technique has required welding at different electrical current levels to achieve the desired sequence of multi-level heat input. For this reason, a welding electrical energy source or power supply has been used which is capable of supplying current at the different levels to accomplish the low heat/high heat sequence. However, the requirement for such power supply has contributed to the high cost and complexity of controlling operation of this technique with the resultant overall difficulty in forming welds with reduced defects.
Consequently, a need exists for improvements in the low heat first past/high heat second pass technique to reduce or overcome these shortcomings.